A longitudinal study of memory and metamemory impairment in multiple sclerosis

  • Marian Scarrabelotti

    Student thesis: Doctoral Thesis

    Abstract

    The symptomatology of Multiple Sclerosis (MS) includes cognitive impairment which can have a major impact on quality of life, particularly on social and occupational competency. Memory disturbance is the most consistently reported cognitive impairment in MS with an estimated incidence of 40-60%. Despite its frequent occurrence and serious implications, memory disturbance in MS cannot be reliably predicted by disease course or duration, neurological disability status, or neuroimaging. Further, there is little agreement about the underlying cause or causes of memory dysfunction, as well as great individual variability in the severity and progression of memory disturbance. The purpose of the present investigation is to understand better the nature of MS memory dysfunction. This was attempted by addressing the serious methodological inconsistencies and inadequacies which have contributed to previous inconclusive memory findings. The key design features of the present study were the use of: a) a longitudinal design with an adequate control group; b) Lublin and Reingold’s (1996) improved MS diagnostic classifications; c) Magnetic Resonance Imaging (MRI) measures to investigate possible neuroimaging/memory correlations; d) statistical means to control the effects of demographic variables on MS memory; and e) subject exclusion criteria to avoid the confounding effects on memory of factors such as depression, pre-existing or concurrent neurological conditions, and drug treatments. An equally important reason for previous inconclusive MS memory findings is the inadequacy of conventional definitions of memory impairment. To address this problem, Jacoby’s (1991) model of memory was used. It assumes that independent conscious and automatic memory processes both contribute to performance in any memory task. This contrasts with conventional memory measures which assume that explicit tests measure conscious processing and implicit tests measure automatic processing. In addition to using this novel measure of memory, MS metamemory (knowledge about one’s own memory) and executive skills were also investigated. It was argued that because metamemory and executive skills rely especially on conscious processing, and previous MS studies have documented impairment in both types of skills, that these variables would be significant in the development of a predictive model of MS conscious memory performance. The addition of information about brain lesion location was also expected to contribute to the predictive power of the model, because recent studies have found significant relationships between MRI variables and neuropsychological test performance. The best variables from the areas of metamemory, executive skill, and neuropathology were to be combined in a model which predicts conscious memory performance. Forty-six MS patients diagnosed according to Lublin and Reingold’s (1996) criteria were compared with 40 control subjects matched on age, sex, and education. There were 3 testing occasions at 12 monthly intervals over 24 months. The MS group were impaired in conscious memory estimates, metamemory judgments, and in several tests of executive skills compared to controls when averaged over 24 months. In the MS group, neuropathology most typically featured lesions in the periventricular regions of the brain. MS and control groups did not differ on automatic memory estimates or on executive tests that relied on well-rehearsed skills. In general, MS cognitive performance (in conscious memory, metamemory, and executive skills) did not deteriorate over 24 months in relation to controls. The exception to this was a significant deterioration in MS group performance in executive control (as measured by the Stroop test), and deterioration over 24 months in conscious memory for a sub-group of 4 MS patients. These 4 MS patients generally had high automatic memory estimates, high total and periventricular lesion loads and increasing subcortical frontal/parietal loads over 24 months, deteriorating Stroop scores, and were typically female, aged 40, with a secondary progressive disease course, and an average of 13 years of formal education. Significant correlations were found between conscious memory estimates and all the metamemory variables (except ‘judgment of learning’ accuracy), and between conscious memory and most of the executive tests used. No significant correlations were found between conscious memory and any of the MRI brain lesion regions. Peri ventricular and total, rather than exclusively subcortical frontal lesion areas were related to performance on several executive tests. The metamemory measure ‘feeling of knowing’ was related to subcortical temporal lesion area. Most importantly, the findings from the present study culminated in the formulation of a memory model. By year 3, the best model developed in the study combined information about the strongest executive and metamemory predictors, and explained 36.7% (Adj. R2 = 31.8%) of memory variance. Executive control (measured by colour-words named in the Stroop test) and metamemory (feeling of knowing) magnitude together accounted for a unique 21.0 % of conscious memory variability. The control variables age, sex, and years of formal education accounted for 14% of the variance. Because brain neuropathology measures were not significantly correlated with MS conscious memory, they were not included in the memory model. Yet, in several sub-groups of the most impaired MS patients, high total and periventricular lesion areas were more consistently prominent than subcortical frontal lesions, compared with the total MS group. The major implications from the research findings for this MS group are discussed. First, from the practitioner’s perspective, the study has produced a brief initial screen for the accurate prediction of conscious memory performance in MS patients at any point in time. Second, the memory model can be further developed using normal control subjects because group differences in memory performance were qualitative. Control subjects generally performed at the less impaired end of the range in tests of conscious memory, metamemory, and executive skills. This has significant consequences for the future development of the model. Most importantly, however, the rehabilitation implications of greater accuracy in MS memory and metamemory measurement are examined. Finally, the benefits of a shift in understanding which conceptualises MS cognitive dysfunction in terms of impaired conscious, intentional processing and intact automatic processing are discussed.
    Date of Award2000
    Original languageEnglish
    SupervisorMarie Carroll (Supervisor) & Anita Mak (Supervisor)

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